Bio-impedance measurement techniques are known in the art. Bio-impedance measurement consists of passing a known current through tissues and measuring the resulting potential difference. Bio-impedance measurements can be used to determine a variety of properties of the tissue under investigation, for example for determining the structure of bone tissue.
To obtain accurate and useful results, it is essential to measure the potentials appearing across the tissues under investigation and not the potential across the tissues which surround the investigated ones, and to ensure that as much as possible of the measuring current flows through the investigated tissue. To achieve this, in the most frequently used configuration, separate electrodes are used to inject measuring current and to measure the resulting potential difference. This method is known as the four-electrode method and it eliminates the influence of contact resistances of measurement electrodes.
When parasitic current flows through tissues which are not under investigation it can create significant problems, resulting in loss of accuracy and reliability of the output measurements. A method of applying a dynamically generated, additional screening potential to the tissues has been proposed, which can greatly reduce undesired current flows. Such a method, using 6 to 8 electrodes, is described in international patent application No. PCT/PL2011/000014, entitled “Method and Apparatus for Non-invasive Analysing the Structure and Chemical Composition of Bone Tissue Eliminating the Influence of Surrounding Tissues.” The purpose of the electronic circuit controlling the electrodes is to generate a measuring current that is both adequate and safe, to measure the resulting potential, and—in the case when a dynamic screening potential is applied—to generate an additional screening potential. As the number of measurement electrodes grows, the complexity level of the control circuit, and the number of pins in the connector between the control circuit and the electrodes, grows as well, but the measurement capabilities of the entire apparatus are increased.
The measurement method described in the above mentioned patent application, in its most sophisticated form, requires 8 measurement electrodes. It consists in conducting measurements as in the classical four-electrode method, with the difference that the value of measured potential is monitored and the value of screening potential is dynamically adjusted so that the undesired current flows are minimized. The screening potential results from the flow of screening current, which typically reach values greater than the measuring current, and in extreme cases may be much greater than the measuring current. Such a method of current control requires that the very low level of measuring current intensity is applied, so that the larger screening current does not exceed the safety threshold defined by medical devices safety regulations. However, the use of too low a measuring current, decreases the signal to noise ratio and lowers the sensitivity of the apparatus. This makes implementing the measurement method difficult in practice.
It is an object of the invention to provide an alternative bio-impedance measurement method and control circuit that allows for greater control over the screening current and measurement current levels. It is also an object of the invention to provide a control circuit that allows for the possibility of changing the measurement configuration by simply changing the connection of electrodes to the control circuit.